This invention relates to industrial baghouses and, more particularly, to an improved modularized baghouse especially adapted for on-line maintenance to permit 24-hour operation.
Continuous emphasis on environmental quality has resulted in increasingly strenuous regulatory controls on industrial emissions. One technique which has proven highly efficient in controlling air pollution has been the separation of undesirable particulate matter from a gas stream by fabric filtration.
Such filtration is carried out in dust collection apparatus known in the trade as a "baghouse" which operates on the same general principle as an ordinary household vacuum cleaner, except on a much larger scale. Basically, the baghouse includes a sheet metal housing divided into two chambers, referred to as plenums, by one or more tube sheets. Disposed within openings communicating with the plenums are fabric filters. A particle-laden gas stream, induced by the action of a fan, flows into one chamber (dirty air plenum) where dust accumulates on the fabric filter as the gas passes through the fabric into the other chamber (clean air plenum) and out the exhaust.
Although all baghouses are designed in accordance with the foregoing general principles, there are numerous operational and structural distinctions. The present invention relates to a modularized baghouse for continuous operation in a high temperature and corrosive environment. For example, boilers, kilns and furnaces may typically burn sulphur-laden coal, creating a corrosive stack gas. Plants of this genre characteristically operate around-the-clock and it is naturally desirable that the air pollution control equipment also be capable of continuous operation. A modularized baghouse made up of several filter modules filtering in parallel is particularly useful for such operation, since the majority of the modules can be left in service while maintenance is performed on those modules out of service.
Each individual module is itself a complete baghouse wherein the dirty and clean air plenums are separated by a tube sheet having a plurality of vertically suspended filter bags in which cylindrical wire cages are inserted for skeletal support. Filtration of the process gas occurs from outside to inside of the bags. As a result, baghouses constructed in this manner are normally referred to as outside bag collectors.
The effectiveness and efficiency of a modularized baghouse is directly related to the manner in which it facilitates on-line maintenance. On occasion, it is necessary to isolate an entire module for routine maintenance and inspection. During such times, it may be necessary to replace only a few bags or the entire removal of the tube sheet and associated bags may be mandatory for a complete changeout of the filters. It is desirable that the tube sheet and associated filters be removed and be replaced as a complete unit as quickly and as conveniently as possible so that the module can be brought on-stream again when needed. When inserting a new filtering unit or replacing the old unit, it is imperative that the bags be maintained in proper vertical alignment to assure proper operation and to prevent premature bag failure. Insurance of this alignment has a tendency to be time-consuming and labor intensive and, therefore, represents a significant maintenance problem.
Another maintenance difficulty resides in effectively sealing the tube sheet against the housing of the module. Failure to do so naturally results in leaks between the dirty and clean air plenums and impairs the operating efficiency of the equipment. Consequently, there is a need for an effective sealing technique which facilitates removal or installation of a complete filter unit in a baghouse module.
There are several methods for cleaning the bags to remove the filter cake which is deposited on the fabric during operation of an outside bag collector. In pulsing plenum cleaning, high pressure air is introduced into the clean air plenum and the dust cake is simultaneously dislodged from all filter bags. With pulse jet cleaning, on the other hand, a short blast of high pressure air is individually introduced to each cleaning bag. This pulse of high pressure air travels through the filter sleeve to dislodge the dust cake.
Further details of a baghouse having cages and bags suspended from a tube sheet and employing pulse jet cleaning may be found in U.S. Pat. No. 3,876,402 by Bundy et al., issued Apr. 8, 1975 and specifically incorporated herein by reference.
Although the foregoing cleaning methods are effective in removing the dust cake, they are not without drawbacks. When using high pressure air, the bags tend to snap back onto the cage after cleaning and this causes excessive wear in some bags. With emphasis on prolonging bag life and reducing maintenance requirements, there is a need for an improved cleaning method having a gentler action on the bags to reduce wear.
Rough cleaning treatment, however, is only one of the many causes of bag wear. The filters also experience excessive wear when directly impinged by a particulate-laden gas stream introduced into the dirty air plenum. In the past, this problem has chiefly been dealt with by baffling the inlet process gas, but the baffles tend to create dead or isolated spots among the bag filters, which impedes efficiency. Consequently, there is need for an effective way of distributing the inlet gas flow to prevent direct impingement of the particulate-laden stream on the filter bags.
As one might expect, there are difficulties associated with determining the appropriate time for initiating the cleaning cycle. The criteria employed is normally based on pressure drop across the tube sheet or, in the case of a modularized installation, in the pressure drop across the inlet and outlet headers. The equipment used for measuring the pressure drop typically includes a pressure differential sensor with associated pressure taps located in the inlet and outlet conduits. The pressure taps are unfortunately prone to plugging and clogging, which results in erroneous readings and generally represents an unsatisfactory basis upon which to control the cleaning process. Accordingly, there is need for an effective and reliable basis on which to initiate the cleaning cycle.
The primary object of this invention is to provide solutions to some of the foregoing problems experienced in the operation and particularly in the maintenance of industrial baghouses.
More specifically, an object of this invention is to prolong, by means of an improved construction and maintenance techniques, the life of the filter fabric used in an industrial baghouse. A correlative object is to provide a baghouse achieving prolonged filter life by reducing wear and abrasion on the filter fabric. At the heart of this goal is an improved cleaning technique adapted to gently clean the filter bags and to minimize the wear associated with vigorous flexure of the filters.
An additional object of the invention is to provide, for a modularized baghouse, a unitized tube sheet and filter construction which rigidly maintains alignment of the filter bags with respect to the tube sheet to thereby facilitate installation and removal. As an adjunct of this object, the improved construction enables preassembled shipment of the unit, since the filtering cartridge may be transported upright or on its side.
Yet another object of the invention is to provide effective means of sealing the filter unit or cartridge within the housing module in order to prevent gas leakage between the dirty and clean air plenums.
A further object of the invention is to minimize bag wear caused by impingement of particulate-laden inlet gas. A diffuser apparatus associated with the inlet to the dirty air plenum directs the incoming gas in such a manner as to prevent direct impingement on the filter bags, but at the same time eliminates any dead spots of filters as heretofore experienced with the use of a baffle construction.
Yet a further object of the invention is to provide an effective technique to initiate the cleaning cycle for a baghouse. Incorporated in such a technique is means for accurately and reliably measuring the pressure drop across the baghouse in order to initiate cleaning. Integrally part of this technique is provision for cleaning the pressure taps employed with a differential pressure sensor prior to the time of any control measurement.
Other and further objects of the invention, together with the features of novelty appurtenant thereto, will appear in the course of the following description of the drawings.